In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
There are two modes by which the operating mechanism for the circuit breaker can disengage the operating contacts: the circuit breaker operating handle can be used to activate the operating mechanism; or a tripping mechanism, responsive to unacceptable levels of current carried by the circuit breaker, can be used to activate the operating mechanism. For many circuit breakers, the operating handle is coupled to the operating mechanism such that when the tripping mechanism activates the operating mechanism to separate the contacts, the operating handle moves to a fault or tripped position.
To engage the operating contacts of the circuit breaker, the circuit breaker operating handle is used to activate the operating mechanism such that the movable contact(s) engage the stationary contact(s). A motor coupled to the circuit breaker operating handle can also be used to engage or disengage the operating contacts. The motor can be remotely operated.
A typical industrial circuit breaker will have a continuous current rating ranging from as low as 15 amps to as high as several thousand amps. The tripping mechanism for the breaker usually consists of a thermal overload release and a magnetic short circuit release. The thermal overload release operates by means of a bi-metalic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as for the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross-section and related elements are specifically selected for such current range resulting in a number of different circuit breakers for each current range. Electronic trip units are also used in some applications.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker. Again, electronic trip units can also be used.
It is well known in the art that a trip unit for a circuit breaker can be what is referred to as a plug-in unit. Typically, the plug-in trip unit contains the thermal magnetic trip mechanism or the electronic trip mechanism electrically coupled to the load terminals for the circuit breaker. The plug-in trip unit is coupled to the frame portion of the circuit breaker. The frame portion of the circuit breaker typically contains the line terminals, the contact arms, the operating mechanism and the handle of the circuit breaker. The frame portion of the circuit breaker may also contain accessories that work in conjunction with the circuit breaker for various applications. The connection between the plug-in trip unit and the circuit breaker frame unit typically are bolted/tab connections or flexible braid connections. However, those types of connections are relatively expensive to manufacture and assemble, therefore manufacturing is relatively time consuming. Another approach to the connection between the plug-in trip unit and the breaker frame unit is to provide a blade that is slidingly engageable with a jaw to permit electrical engagement and disengagement between the trip unit and the frame unit. However, this arrangement creates significant problems of tolerances in trying to maintain good electrical contact between the contact arm, the connectors and the trip unit stabs. Each part in the assembly is subject to manufacturing tolerances that cause variation in the positions of the parts. In multi-pole circuit breakers such tolerance problems are multiplied by the number of poles in the circuit breaker. Another problem encountered in the blade and jaw arrangement is that the flat mating surfaces can cause excessively high and variable resistances in the joint as well as concentrating a high fault current in a relatively small area thereby causing current constriction forces which tend to blow the joint apart and cause undesireable arcing in the joint. Additional problems with such an arrangement include excessive friction which can result in either an inhibited ability to plug in the trip unit or inhibit the rotation of the contact arms.
Thus, there is a need for a molded case circuit breaker that provides a plug-in trip unit joint that facilitates an easy connection and disconnection of the trip unit from the circuit breaker frame unit. There is a further need for a plug-in trip unit joint that reduces or eliminates positional and angular misalignment between the contact arms and the trip unit while maintaining the functional quality of the electrical interface. There is an additional need to provide a plug-in trip unit joint that minimizes frictional torque on the movable contact arm of the circuit breaker and to not excessively concentrate the electrical current in the joint.